refactor(client): shared declaration-aware registration deferral

The five settings-surface registrants carried near-identical
spec-check/ledger-judge/subscribe scaffolding (three jscpd clones);
ui-slots now owns deferRegistration() — ledger-judged presence, refresh
for registrant-localized labels, one-call disposal — and every
registrant shrinks to its registration body.

packages/client/ui-slots/src/index.ts

@@ -18,6 +18,7 @@ import type { BoundActions, HandleOf, PropsStore, SnapshotSelectorHook, StoreDec
 
 export * from './store.ts'
 export * from './renderer.ts'
+export * from './deferred.ts'
 
 /** Slot contract table. Owners extend via declaration merging; entries are {@link SlotEntryDef}. */
 export interface SlotMap {}